Video waveform monitors, for example the Tektronix 1740 and 1750 series and the digital video waveform monitor VM700, are known. Such monitors are specialized and relatively expensive test equipment meant for analyzing video signals. These monitors have the capability to display video waveforms and vector diagrams. In addition, they have the capability to display images represented by video signals, thus allowing the them to act as video monitors. This may be done, for example, by sampling the received video signal, then generating from the video samples an array of display pixels representing the image carried by the video signal. This array of display pixels is displayed on a display device associated with the video waveform monitor. However, such a capability requires a high sampling rate and large acquisition memory to store the video samples representing a frame of the image (sometimes termed record length). These requirements were met only in the relatively expensive, high-end, specialized video waveform monitors. It is desirable, however, to provide the video image display capability in general purpose digital storage oscilloscopes.
One problem in producing an image representing a video signal is to extract the gray scale (luminance) and color (chrominance) components from the composite video signal. For low-end systems, gray scale images only are displayed. This requires that a minimum of the luminance component be extracted from the composite video signal.
One known method for extracting the luminance and chrominance components is to oversample the video signal sufficiently that digital filters may filter out the luminance and chrominance components. These filters may be implemented on a digital signal processor (DSP) integrated circuit chip. They may be arranged as a low pass filter and a bandpass filter in parallel processing the composite video signal: the low pass filter extracting the luminance component and the bandpass filter extracting the chrominance component. Alternatively, the filters may be implemented as a comb filter processing the composite video signal which simultaneously extracts the luminance and chrominance components. In either arrangement, this method requires a high sample rate, and consequently has a large record length requiring a large memory to store a video frame of samples.
Another method is to use a hardware probe module containing a discrete analog low pass filter which extracts the luminance component. This method does not affect the sample rate of the oscilloscope, but requires a separate hardware assembly, which must be removed from the oscilloscope probe should a user wish to observe the color content of the video signal.
Another method is to use a high sampling rate and generate small display pixels corresponding to the video samples without any processing to display the received video. Because of the phasing of the color subcarrier, the human eye will see a dithered pattern of light and dark pixels and will tend to average the brightness of those pixels to the underlying luminance level underlying the chrominance component. It is also possible to perform minimal processing of the display pixels, for example by averaging two adjacent pixels, to improve the displayed image. Because of the high sampling rate, this method may also require a large record length and consequent large memory.